The present invention relates to air interface design, and more particularly to the use of frequency hopping in a bandwidth-on-demand communications system
In an orthogonal frequency-division multiple access (OFDM) system, the radio band is divided into many narrow orthogonal sub-bands, or channels. In such a system, bandwidth requirements can be tailored to suit each individual user ("bandwidth on demand") by assigning to each user as many channels as he or she requires.
In an unlicensed band, or in a situation in which channel allocation is uncoordinated, one or more channels may be jammed by other systems or uncoordinated users. One way to reduce the impact of such interference is to use frequency hopping (FH) techniques, where the carrier frequency is changed frequently so as to avoid being jammed continuously. Well-known coding, interleaving and/or retransmission techniques may then successfully be applied to reduce the interference problem.
The combination of OFDM with frequency hopping has the promise of providing bandwidth on demand and jamming resistance. When, however, much of the available bandwidth is allocated to one or only a few users, there are no unused frequencies to hop to. As a consequence, frequency hopping is ineffective under these circumstances. For example, if half of the channels are allocated to one user in a block of channels, there is only one other block to hop to. Similarly, if one fourth of the channels are allocated in a block, there are only three unused blocks of frequencies to choose from for the next hop.
There is therefore a need for techniques that will permit OFDM and other modulation schemes to offer bandwidth-on-demand in combination with effective frequency hopping techniques.